1. Field of the Invention
This invention relates to a piston ring for an internal combustion engine and, in particular, to an oil ring having a coil expander for scraping excess oil from a cylinder wall.
2. Description of the Prior Art
In general, piston rings are placed between pistons and inner walls of engine cylinders. In automobile engines, these piston rings usually include one or two oil rings per piston for scraping excess oil from the cylinder walls. An oil ring must always bear against the cylinder wall. For this purpose, an oil ring is provided with a coil expander which forces the associated oil ring out against the cylinder wall.
An oil ring usually consists of upper and lower side rails and a web connecting the two side rails. The side rails and the web define an outer peripheral channel of the oil ring for receiving the scraped oil. They also define an inner peripheral groove of the oil ring for receiving the annular coil expander. The web of the oil ring is provided with a plurality of elongated oil vent holes which allow scraped oil to pass from the outer peripheral channel into the inner peripheral groove.
The scraped oil passes from the inner peripheral groove and piston peripheral groove into the piston through oil drain holes provided in a peripheral groove of the piston. The oil is then returned to an oil pan of an engine.
In the prior art, the inner peripheral groove of the oil ring, which is usually made of cast iron, usually has had an archwise sectional shape complementary to the circular sectional profile of the coil expander. As a result, the coil expander cam into considerable surface contact with the inner peripheral groove. Too much surface contact between the coil expander and the oil ring, however closes the oil vent holes to a large extent. This prevents the smooth passage of the oil scraped by the oil ring from the outer peripheral channel to the inner peripheral groove, i.e., into the peripheral groove of the piston. In some cases, the oil vent holes are completely clogged with oil sludge. This consequently reduces the oil scraping effect of the oil ring. The decrease of the oil scraping effect results in an increse of oil consumption.
Further, as the conventional oil ring has been usually made of cast iron, it could not be made light in weight, because the lower strength of cast iron meant the thickness of the web of the oil ring could not be decreased. Furthermore, in case of a cast iron oil ring having a coil expander which comes into considerable surface contact with the archwise periphery of the oil ring, the coil expander may be thrust totally into the archwise periphery of the oil ring due to wear of the oil ring.